A scanned PDF is really pictures of pages: there’s no text to select or search. When you open one, Seben shows a banner offering to Run OCR.
Click it and Seben recognizes the text on every scanned page. You can then select and search that text in Seben’s viewer, and search highlights matches on the scanned page.
Run OCR after your other changes. The recognized text belongs to the version OCR created, and a later change makes a new version without it.
Export to Word & Excel recognizes scanned pages on its own, so you don’t need to run OCR first.